In recent years, a fuel cell is attracting attention as a clean energy source. One example of the fuel cell is a polymer electrolyte fuel cell. The polymer electrolyte fuel cell (hereinafter referred to as “PEFC”) includes a membrane-electrode assembly, and an anode separator and a cathode separator disposed to sandwich the membrane-electrode assembly and respectively contact an anode and a cathode. The membrane-electrode assembly includes the anode and the cathode (each of which is referred to as “electrode”) each constituted by a gas diffusion layer and a catalyst layer. The gas diffusion layer has fine holes that are flow paths of a reactant gas. A fuel gas channel is formed on one main surface of the anode separator. An oxidizing gas channel is formed on one main surface of the cathode separator. A fuel gas (hydrogen) supplied through the fuel gas channel to the anode is ionized (H+), flows through the gas diffusion layer and catalyst layer of the anode, further flows through the polymer electrolyte membrane via water, and moves to the cathode. The hydrogen ion having reached the cathode generates water through the following electric power generating reaction in the catalyst layer of the cathode.Anode: H2→2H++2e−Cathode: (½)O2+2H++2e−→H2OTotal Reaction: H2+(½)O2→H2O
The water (generated water) generated as above flows to the oxidizing gas channel of the cathode separator as steam or liquid. Moreover, a part of the water generated in the cathode moves to the anode (so-called “back diffusion”). Therefore, as each of the oxidizing gas and the fuel gas (each of which is referred to as “reactant gas”) flows from an upstream portion to downstream portion of each of the oxidizing gas channel and the fuel gas channel, a partial pressure of steam in each of the oxidizing gas and the fuel gas increases. With this, especially when the fuel cell is driven at high temperature and high humidity (for example, the dew point of the reactant gas is set to be the same as the temperature inside the fuel cell stack), flooding occurs by clogging of the generated water in the downstream portion of the oxidizing gas channel or the fuel gas channel or by clogging of the generated water in the fine holes of the gas diffusion layer opposed to the oxidizing gas channel or the fuel gas channel.
Disclosed as one example of a technology for suppressing the occurrence of the flooding is a fuel cell in which at least one of the depth and width of the oxidizing gas channel is gradually reduced from an upstream channel region to downstream channel region of the oxidizing gas channel (see Patent Document 1 for example). In accordance with such fuel cell, the flow velocity of the oxidizing gas flowing through the downstream channel region of the oxidizing gas channel increases, so that the generated water clogged in the oxidizing gas channel can be discharged.
Patent Document 1: Japanese Laid-Open Patent Application Publication 6-267564